The pulping system described in U.S. Pat. Nos. 5,489,363; 5,536,366; 5,547,012; 5,575,890; 5,620,562; and 5,662,775 and marketed under the trademark LO-SOLIDS.RTM. pulping by Ahistrom Machinery Inc., of Glens Falls, N.Y., has revolutionized the art of chemical pulping. Among other things, the many manifestations of the LO-SOLIDS process has introduced a marked increase in the versatility of operating continuous pulping systems. However, in addition to the versatility that the LO-SOLIDS process provides for the pulping process, this process also introduces versatility to the cooking system and in particular to the vessels and other equipment associated with the pulping process.
It is well established that the counter-current flow of cooking liquors within a continuous digester vessel limits how much cellulose material can be processed in the vessel. In the treatment of comminuted cellulosic fibrous material in a vertical vessel, the driving force for the downward flow of material is gravity. The downward force of gravity is opposed by the buoyant force of the material in the liquid. The typically well-impregnated material is denser than the surrounding liquid and the material "sinks" in the vessel. However, the difference in density of the material and the surrounding liquor is not great; the difference in specific gravity between a typical fully-impregnated, gas-free wood chip and that of the surrounding liquid is only about 0.1 to 0.2. Thus any additional forces that tend to counter-act the force of gravity can interfere with the flow of material through the vessel.
One such force which opposes the downward flow is friction, specifically the friction imposed by the internal surfaces of the vessel. Screen assemblies through which liquid is removed from the material slurry provide a significant source of friction to the downward movement of the cellulose material. The typically lateral flow of liquid toward the annular screen assembly imposes a normal force on the screen assembly which locally increases the force of friction at the screen. If the friction force at the screen exceeds the downward force of the chips, the flow of chips can be hindered, or "hung up" on the screen assembly, with adverse consequences for uniformity of treatment.
Another force that acts upon the chip mass in a continuous digester (the chip mass is often referred to as the "chip column") is the viscose force of liquid flowing upward or downward in the chip column. Downward, or co-current, flowing liquids aid the force of gravity and promote the downward flow of material. Upward, or counter-current, flowing liquids counter-act the force of gravity. In extreme cases, the force of the upwardly flowing liquid can impede the downward flow of material.
The upward force on the chip column due to the upward flowing liquid is dependent upon the upward flow velocity of the liquid. The higher the flow velocity, the higher the resistance to downward flow and the more the resilient chip column is compacted. This compaction further impedes the upward flow of liquid. The flow velocity is dependent upon the production rate or the amount of cellulose material passing through the vessel per unit time. As the production rate increases the flow required to maintain an upward counter-current displacement of liquid increases. Typically, the resistance and compaction imposed by the counter-current flow of liquid limits how much production can be passed through the vessel. A point can be reached where the resistance and compaction caused by the upward flow of liquid overcomes the downward force of gravity and the downward flow of material is impeded or stopped entirely.
In one embodiment of this invention, this limitation upon the production rate that can be passed through a treatment vessel is substantially eliminated by substantially eliminating or minimizing the need for the counter-current flow of liquors within the chip column. In another embodiment of this invention the size of a continuous digester vessel can be reduced by eliminating the need for one or more countercurrent treatments while still effecting the desired treatment of the pulp.
In conventional continuous digesters, the ratio of length, L, of a vessel to its diameter, D, typically varies between 7 and 9. However, using the present invention the design of continuous digesters is not limited to this conventional range of UD and vessels having larger or smaller L/D ratios may be used. This latitude in vessel dimension provides the designer much more flexibility in designing and constructing digester vessels specifically, and pulping systems in general.
The present invention can be used in situations where space is a limiting factor and a narrower, taller vessel is desired. Also, the present invention is advantageous when expanding the capacity of existing, typically older, narrower digesters where counter-current treatment is not possible or undesirable. The present invention is also advantageous when a wider vessel can be used, for example, when space is not a limiting factor. Again, according to the present invention, L/D ratios larger and smaller than are conventional can be used.
In addition, the present invention is also applicable to zones in a vessel that are production limited due to their geometry. Where the discussion above discusses the dimensions of a vessel as a whole, the present invention is also applicable to cylindrical treatment zones in the digester, for example cooking zones or washing zones, that are production limited or exhibit less than optimum treatment conditions due to their geometry. The production and treatment in these zones can be enhanced by introducing "cross-flow" treatment or by substituting cross-flow treatment for counter-current treatment.
Conventionally the control of temperature and chemical distribution throughout the height of a continuous digester requires that liquors be removed from the vessel by several screen assemblies, augmented with cooking or dilution liquor, and returned to the digester, typically after passing through a heat exchanger. Such typical liquor removal and recirculations are shown in published EP patent application 476,230 which discloses a process marketed by Ahlstrom Machinery under the name EMCC.RTM. cooking, or in the US patents identified above for LO-SOLIDS cooking. However, these liquor removal and re-introduction circulations typically require individual pumps and heat exchangers as shown in these patents and the published EP patent application. Typically, the increased process versatility provided by these processes also require additional equipment, in particular additional pumps and heat exchangers. However, the unique features of LO-SOLIDS pulping, in which multiple extractions of cooking liquor having a high concentration of dissolved organic material and replacement of the cooking liquor with fresh cooking liquor and liquor having a lower concentration of dissolved organic material, provides the opportunity of reducing the number of pumps, heat exchangers and other equipment, required to continuously produce chemical pulp.
This is especially true since the liquids introduced to the cooking circulations in the LO-SOLIDS process are relatively uniform in composition. Where in conventional processes the concentration of cooking chemical and the concentration of dissolved material introduced at multiple locations typically varies significantly, the concentration of liquids introduced at two or more locations using the LO-SOLIDS process may be fairly uniform. For example, the liquid introduced at two or more locations in the LO-SOLIDS process may typically have a dissolved solids concentration of 60-80 g/l and an effective alkali (EA) of 12-20 g/l (as NaOH). Thus, using a single pump and associated equipment to distribute these two or more streams of liquid becomes particularly viable when employing the LO-SOLIDS process.
One embodiment of this invention comprises or consists of a method and apparatus for treating comminuted cellulosic fibrous material in which the production rate is not limited by the counter-current flow of liquid during treatment. According to this aspect of the invention a method of treating comminuted cellulosic fibrous material (such as wood chips) in a substantially upright vessel (such as a continuous digester) is provided. The method comprises substantially continuously: (a) feeding a liquid slurry of comminuted cellulosic fibrous material into a top portion of the vessel; (b) withdrawing treated comminuted cellulosic fibrous material from a bottom portion of the vessel, so that the comminuted cellulosic fibrous material moves substantially continuously downwardly in the vessel; and (c) in substantially the lower half of the vessel, causing treating liquid to flow substantially horizontally into and out of contact with the comminuted cellulosic fibrous material as the comminuted cellulosic fibrous material flows downwardly in the vessel, to effect treatment of the comminuted cellulosic fibrous material substantially without interfering with the substantially continuously downward movement of the comminuted cellulosic fibrous material (i.e. so that there are substantially no vertical counter-current zones).
Preferably (c) is practiced using at least one screen assembly at a peripheral portion of the vessel, and by introducing treating liquid into a central portion of the vessel at the approximate level of the or each screen assembly, and withdrawing the majority of the liquid introduced at the approximate level of the or any screen assembly through that screen assembly. Also step (c) may be practiced at a plurality of vertically spaced screen assemblies, and (c) is typically further practiced using cooking liquor (such as kraft cooking liquor like white liquor) as the treating liquid at at least one of the screen assemblies (and as many as all), and (c) may also be further practiced by recirculating liquid withdrawn from one screen assembly as introduced treating liquid at a central portion of another screen assembly (such as the immediately above screen assembly). Also (c) may be practiced by using one or more screen assemblies during or after the cellulose material is exposed to cooking temperature, e.g. 140.degree. C. or greater.
The method may further comprise (d) augmenting the flow of liquid introduced at at least one central portion with a non-cooking liquor having a dissolved organic material concentration less than 1/2+L (desirably substantially zero) that of the dissolved organic material concentration of the liquid withdrawn through the screen assembly associated with that central portion. For example (d) may be practiced using washer filtrate.
In the preferred embodiment the vessel is substantially cylindrical and has an L/D ratio (the length or height to the width or diameter, in the case of a circular cylindrical vessel, the diameter outside the ratio 7-9 to 1); and (c) may be practiced by causing the treating liquid to flow substantially radially in the vessel. (c) may be practiced at two, three or more vertically spaced locations along the substantially upright vessel and using a cooking liquor as the treatment liquor at at least two of the locations. (c) may also be further practiced by recirculating radially flowing liquid at one location as introduced induction liquid at another location, such as the immediately vertically above location.
According to another aspect of the invention an apparatus for treating comminuted cellulosic fibrous material is provided. The apparatus comprises the following components: A substantially upright vessel having a top and a bottom, a length or height L, a maximum width or diameter D, an upper half, and a lower half. An inlet for a liquid slurry of comminuted cellulosic fibrous material adjacent the top of the vessel. An outlet for a treated liquid slurry of comminuted cellulosic fibrous material adjacent the bottom of the vessel. At least first and second vertically spaced screen assemblies disposed within the vessel. At least first and second liquid introducing structures disposed at the approximate vertical level of each of the screen assemblies which introduce treating liquid that is ultimately withdrawn through the first and second screen assemblies, respectively. The UD ratio is preferably outside the range of 7-9 to 1, e.g. about 10 to 1 or greater, or about 6 to 1 or less. The vessel is substantially devoid of internal counter-current flow between materials and fluids (particularly liquids) in substantially the lower half of the vessel.
Preferably the vessel comprises a continuous digester, and also preferably it is substantially cylindrical. The vessel may have one or more step outs along its length, or it may also have no step outs, and may have at most one step out.
The first screen preferably is above the second screen and the apparatus further comprises means for recirculating liquid withdrawn from the second screen to the second liquid introducing structure. Liquid introducing structures may be open-ended or perforated along their length, such as in conventional digesters, or any other suitable conventional structure capable of properly introducing liquid. The at least first and second screens may comprise at least first, second, third and fourth screens, and preferably the at least first and second liquid introducing structures comprise at least first, second, third and fourth liquid introducing structures. The apparatus may further comprise liquid recirculating means for recirculating liquid from at least another screen besides the second screen to another liquid introducing structure besides the first liquid introducing structure.
The second, third or fourth screen may also be below the first screen. For instance, the first screen may include a circulation and some of the liquid may be removed from the circulation and directed to a lower screen circulation. For example, the first screen may be associated with a cooking or heating circulation in the upper part of the digester and the second screen may be an extraction screen below the first screen. The liquid from the upper circulation, containing, for example, an EA of 15-20 g/l, may be introduced to the vicinity of the extraction screen in, what is termed in the art, the "quench circulation" associated with the extraction screen.
Another embodiment of this invention comprises or consists of a method and apparatus for treating comminuted cellulosic fibrous material in which the number of pumps and other equipment required is reduced compared to conventional systems. In the broadest embodiment of this invention at least two circulations for a continuous digesters are handled by a single pump. According to this embodiment of the invention a continuous digester is provided comprising the following components: A substantially upright continuous digester vessel having a top and a bottom. An inlet for a liquid slurry of comminuted cellulosic fibrous material adjacent the top of the vessel. An outlet for a treated liquid slurry of comminuted cellulosic fibrous material adjacent the bottom of the vessel. At least first and second vertically spaced screen assemblies disposed within the vessel. At least first and second liquid introducing structures which introduce treating liquid that is ultimately withdrawn through the first and second screen assemblies, respectively. Recirculating means for recirculating liquid from the screen assemblies to the liquid introducing structures. And wherein the recirculating means comprises a single pump common to both the first and second screen assemblies and liquid introducing structures.
It is a primary object of the present invention to provide a simplified yet more effective chemical (e.g. kraft) pulp producing method and/or apparatus. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.